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Ringtail Surgeonfish Acanthurus blochii (Valenciennes, 1835)
The Hawaiian fishes of this family are readily identified by having 1 or 2 spines on the side of the caudal peduncle. The species of the genera Acanthurus, Ctenochaetus, and Zebrasoma have a single lancet-like spine which folds into a groove; whereas those of the genus Naso have 2 fixed keel-like spines.
All surgeonfishes have a deep compressed body with the eye high on the head; the mouth is small with a single row of close-set teeth; the teeth may be spatulate with denticulate edges and fixed in the jaws (as in Acanthurus), or numerous, slender with incurved tips, and movable (as in Ctenochaetus).
There is a single unnotched dorsal fin with 4 to 9 spines; the anal fin has 3 spines (except Naso with 2); the pelvic fins have a spine and 3 or 5 soft rays (3 in Naso). Some of the species of Naso have a horn-like projection on the forehead, the basis for their common name unicornfishes (others of the genus without the rostral horn are still called unicornfishes).
The species of Ctenochaetus and several of Acanthurus, such as A. xanthopterus and related species, have a thick-walled gizzardlike stomach. All of the surgeonfishes have a very long intestine. Most feed on benthic algae. The species of Acanthurus graze principally on filamentous algae; those with gizzard-like stomachs often ingest sand with their algal food to assist in trituration of the algae.
Some species of Acanthurus, such as A. triostegus and A. leucopareius, form feeding aggregations to overwhelm territorial herbivorous damselfishes. The algal-feeding species of Naso, N. lituratus and N. unicornis, browse mainly on leafy algae such as Sargassum. The species of Ctenochaetus feed on detritus and algal fragments which they whisk with their comb-like teeth from the substratum (at the same time employing suction). One Hawaiian species of Acanthurus and five of Naso feed mainly on zooplankton well above the bottom.
Surgeonfishes are able to slash other fishes (or humans who do not handle them carefully) with their caudal spines by a rapid sidesweep of the tail; some species have bright hues around the caudal spines as warning coloration.
Spawning from aggregations has been observed in some species of Acanthurus, Ctenochaetus, and Zebrasoma at dusk (and for some at dawn, hence correlated with low light intensity). The eggs are small and pelagic. The late larval stage is orbicular, transparent with silvery over the abdomen, and small scales in narrow vertical ridges on the body. This stage has venomous second dorsal, second anal, and pelvic spines; these spines remain venomous in the adults of at least some of the species of Naso.
Twenty-four species of surgeonfishes are recorded from Hawai'i; however, the colorful Acanthurus lineatus, is known only from two individuals.
Acanthurus dussumieri (Valenciennes, 1835) Palani
Yellowish brown with irregular narrow blue lines on body; head yellowish with blue spots and lines; a broad yellow band across interorbital space, and a yellow spot behind and adjacent to eye; sheath of caudal spine white the socket edged in black; dorsal and anal fins yellow with a blue band at base; caudal fin blue with small blackish spots; dorsal spines 9, rays 25-27. To 18 inches (46 cm). Hawaii and Line Islands to East Africa.
Ctenochaetus strigosus (Bennett, 1828) Kole
Brown with numerous light blue longitudinal lines which extend diagonally into soft part of dorsal and anal fins; a distinct yellow ring around eye; faint blue dots on head; juveniles yellow to yellowish brown; dorsal spines 8, rays 25-28; caudal fin moderately emarginate.
Largest in Hawaii, 7.2 inches (18.3 cm). Indo-Pacific, but with population differences over the range; common in Hawaiian Islands.
Acanthurus blochii (Valenciennes, 1835) Pualu
Dark bluish or greenish gray with small faint yellowish to light gray spots forming irregular longitudinal lines on body; an elongate yellow spot behind eye; a white bar usually across caudal-fin base; dorsal fin dull orange-yellow with 8 or 9 blue bands; dorsal spines 9, rays 25-27. Attains 17 inches (43 cm). Indo-Pacific; feeds more over sandy areas than on reefs.
Naso annulatus (Quoy & Gaimard, 1825)
Dark brown to pale bluish gray; margin of median fins white, edge of lips broadly white, the caudal-fin membranes of large adults also white; juveniles with a white ring around caudal peduncle; adults with a long slender bony horn in front of eye; males with a filament from each corner of caudal fin; dorsal spines 5-6, rays 28-29.To about 3 feet (92 cm).
Indo-Pacific; rare in Hawai'i. Adults generally in more than 100 feet (30.5 m); often in small aggregations. Naso herrei Smith is a synonym.
All information and pictures in this section are from John E. Randall's Shore Fishes of Hawai'i by permission of the author.
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